The objectives of this proposed research are: (1) To determine the significance of the large changes in renin substrate, aldosterone, and blood volume in pregnancy. (2) To determine the role of the renin angiotensin system in uterine function. (3) To determine the mechanisms responsible for the blocking of the biological effects of aldosterone in pregnancy. (4) To determine the mechanisms responsible for the decrease in blood pressure during pregnancy in renal hypertensive and spontaneous hypertensive animals. Acute infusion of an agent which blocks the conversion of angiotensin I to angiotensin II will be used to determine the extent to which the blood pressure of the pregnant animal is being supported by the direct pressor effect of angiotensin. If it has a pronounced effect the blood volume will be expanded and the experiments repeated. The effect of this converting enzyme antagonist on uterine renin and uterine tone will be studied. The Na/K ratio of stool will be used as the indicator of non-renal aldosterone effect. Progesterone will be studied for its ability to inhibit the stool effect of aldosterone. The fall in blood pressure in hypertensive pregnant animals could be due to a vasodilator action of hormones of pregnancy. The effect (singly and in combination) of progesterone, estradiol, prolactin, and chorionic somatic mammatrophin on blood pressure of hypertensive animals will be studied in an attempt to elucidate the mechanisms responsible for the decrease in blood pressure during pregnancy in experimental hypertension.